This is a continuation of PCT application PCT/EP 90/01748 filed Oct. 16, 1990 by Harald Roth and titled "Flourescent Tube Heating And Starting Circuit".
This invention is directed to a circuit for heating and starting a fluorescent tube which can be used for back illuminating a liquid crystal display device.
A prior art circuit for turning on a fluorescent lamp used, for example, to back-light a liquid crystal display has three stages of operation: warm-up, ignition and normal operating. During the warm-up stage the electrodes of the tube are pre-warmed by a current provided by a heating winding. The high voltage needed to fully operate the tube is not switched on during the warm-up stage. In the ignition stage, a voltage which is sufficiently high to cause the ignition of the tube, is switched on. At this time the heating current decreases to a low value close to zero. In the normal operating stage, the high voltage decreases to a value lower than the ignition voltage and the heating current further decreases and can go to zero. In the prior art circuit, a high voltage switch is used on the secondary side of a high voltage transformer to switch the high voltage on at the start of the ignition phase. A high voltage switch of this type is relatively expensive and prone to interference because of the necessary high blocking voltage resistance. A circuit of the type described above is described in GB-A2 212 995. This circuit includes a first transformer having a series connection of two primary windings. The midpoint of the series connection is connected to an operating voltage and the ends of the windings are connected to two power transistors. A secondary winding supplies high voltage for the tube. A control circuit controls the power transistors by two alternating voltages.
The invention is advantageous over the prior art circuits in by the provision of a fluorescent tube starting circuit which operates with the three stages of warm-up, ignition and operation and which eliminates the need for a high voltage switch.